Yep, I'm totally agree! Sometimes it confuses me. I ran into no such file or directory errors in my project and then I remember f4kig placeholder file ?
Sometimes its not clear what you've synced or not during selective sync and its not so easy to figure out.

Hello Stefan,
I've a similar setup to yours and have similar problems. If you've installed sync without messing the configuration, you did everything right. On my NAS Resilio is also much slower than my laptop or computer. I can't give you a clear answer, but maybe it helps. Here are some of my values I recorded.
My NAS max transfer speed is 100MB/s (Local Network)
NAS ------ sync -----> Laptop (37MB/s)
Laptop ------ sync ------> NAS (11MB/s)
You can reconfigure resilio as Gane recommands, but in my opinion it doesn't matter, nothing changed... BUT when I looked at my NAS while transferring data, my NAS says that I'm transferring the data with 42MB/s not 11MB/s as Resilio said. Even if I look on both resilio windows while I'm transferring data I got two different speed values. I don't know If Resilio calculates the transfer speed correctly. I trust my NAS I tested it with copying data without Resilio and checked the speed and it matches perfect.
How much memory do your NAS have? My NAS has 512MB of RAM, so its totally clear for me that my NAS is constant slower. I can see that Resilio eats all the memory while syncing.
I also own a VPS as always online host to avoid relay server (I recommend that for you too). My VPS is 4xCores 16GB of RAM AND no performance issue, the VPS behaves as my laptop or computer.

By default Resilio Sync comes with a default user rslsync. If you want to use sync in your home directory, you've to grant permissions as Gane said. In my opinion to work seamless on a Linux desktop I switched the user. Now Resilio runs as my default working user. No permission trouble at all. https://medium.com/@justlaputa/run-resiliosync-btsync-as-normal-user-under-ubuntu-1498f7701a28

You can share Files with resilio. Thats what I do. If I make a Video, or other Apps have some data you can share nearly any Data on the iPhone (Share via Twitter, Email WhatsApp AND Resilio). In this way any file can find the way into the sync network. I know its not automatically, and not what you looking for but as a hint.

Hello, the new Version isn't in the Linux repositories. My ubuntu installation says there is a new version available.
$ apt show resilio-sync
Package: resilio-sync
Version: 2.5.12-1
Priority: extra
Section: net
Maintainer: Resilio, Inc. <support@resilio.com>
Installed-Size: 16,4 MB
Conflicts: btsync
Replaces: btsync
Homepage: https://resilio.com
Download-Size: 10,5 MB
APT-Manual-Installed: yes
APT-Sources: http://linux-packages.resilio.com/resilio-sync/deb resilio-sync/non-free amd64 Packages
Description: Resilio Sync is a proprietary peer-to-peer file synchronisation tool.
It can sync files between devices on a local network, or between remote
devices over the Internet via a modified version of the BitTorrent protocol.
When does the update comes to the repositories?

And with a little bit more power:
Works on Linux and Mac. And even on Windows if you've installed the Windows Subsystem for Linux.
find . -type f | grep -v .rsls* | awk -F"/" '{print $2"\t"$3"\t"$4}' | sort | uniq
What are we doing here. Get all files from "." directory. Filter all files that don't have the extension ".rsls*" (other files are synced). Print out the second (the first is always a dot) and the third and fourth part. The delimiter is a TAB. After that we will sort it and only get unique. The $n vars are your depth, so in this case you are printing out three parts of the path beginning with the second.